In Star Wars Episode IV: A New Hope, Luke Skywalker knew Obi-Wan Kenobi under the name of "Ben" :

> Well, I stumbled across a recording while I was cleaning him. He says that he belongs to someone named Obi-Wan Kenobi. I thought he might have meant old Ben. Do you know what he's talking about? 

Where did this "Ben" nickname/alias/false identity come from?